This invention relates generally to RF and power transistors, and more particularly the invention relates to a feed bus for such transistors which reduces thermal gradients.
FIG. 1 is a schematic of a bipolar NPN transistor having emitter and collector regions separated by a base region. Applying Kirchoff's current law, emitter current, I.sub.e, equals the sum of base current, Ib, and collector current, I.sub.c.
Typically, emitter and base contacts are provided on one (the front) surface of the transistor and the collector contact is made to the opposing (the back) surface as shown in FIG. 2, with base and emitter wire bonding pads positioned centrally in the metal contact structure and with current flowing to and from interdigitated finger contacts as shown.
The bipolar transistor current relationships introduce mutual magnetic coupling due to magnetic flux linkage between currents. As illustrated in FIG. 3, parallel currents flowing in the same direction produce negative magnetic coupling with the currents tending to oppose each other. However, when the parallel currents flow in opposite directions as shown in FIG. 4, mutual coupling is enhanced or positive and the currents tend to support each other.
Referring again to the conventional bipolar contact structure shown in FIG. 2, current Ib1 receives positive coupling from both Ic and Iel. Current Ib2 receives positive coupling from Ie2 and negative coupling from Ic. Since current proximity is very important to these coupling effects, the dominant mode occurs when Ib couples to Ic. Consequently, the transistor contact structure shown in FIG. 2 will most often run hot on the output side where maximum coupling occurs, unless unusual and deleterious compensations are employed. The transistor base current (the "controlling" current) is enhanced (with positive coupling) which then continuously increases emitter and collector current and consequent heating. If the resulting thermal gradient becomes sufficiently high, then efficiency, linearity, and power output will suffer and eventually the transistor may destroy itself.
A common compensation is to increase transistor "ballasting" (series emitter resistors for sites and fingers) to swamp or at least reduce the transistor output overheating effect. Although increased ballasting improves the current sharing and uniformity, transistor gain and frequency performance suffer significantly if ballasting sufficient to minimize overheating is utilized. Without increased ballasting, load mismatch ruggedness and stability are severely compromised. Such a transistor without adequate compensation will be prone to thermal runaway and may destroy itself. This tradeoff has been a singularly difficult problem with high power, higher frequency devices.